MANILA, Feb 3, 2010 (AFP) - A politician accused of massacring dozens of people in the southern Philippines denied more murders Wednesday as prosecutors charged him with the deaths of 16 other people.

Andal Ampatuan Jr., a member of a politically powerful Muslim clan that was once allied to President Gloria Arroyo, impassively entered fresh pleas of "not guilty" as relatives of some of the dead wept outside the courtroom.

He now stands accused of ordering his armed followers to kill about 57 people in the southern province of Maguindanao on November 23 last year in order to prevent a rival from running against him in May elections.

State prosecutors said some of the cases had only been filed this week because of difficulties finding investigators who were outside the influence of the defendant's powerful family.

The dead included Ampatuan's rival's wife and pregnant sister, as well as 30 journalists.

At the trial Wednesday the defendant, who is in his mid-40s, wore a blue shirt as he talked with his lawyers.

Earlier, Ampatuan's rival, Esmael Mangudadatu, testified that he had previously been warned by Arroyo and her chief aides that the Ampatuans were "dangerous" and that he should not cross them.

He also said that the military and police had refused to give security to his relatives and the journalists accompanying them as they went to file his candidacy.

Prosecutors allege the defendant and up to 100 members of his private army stopped the convoy on a highway in Maguindanao, kidnapped the victims at gunpoint and took them to a hillside where they were shot dead.

They were then buried in mass graves, dug by a government-supplied backhoe, intended for civil works projects.

Mangudadatu told the court that he personally called Arroyo by mobile phone the day after the massacre to inform her what happened. He did not say what her reaction was.

"I could not believe the highest office of the government could not serve or protect the people," Myrna Reblando, the widow of one of the slain journalists, said.

"There were intelligence reports saying this would happen, that these people would be waylaid. She could have done something. These were her people," she said.

Asked if she was referring to Arroyo, she sobbed: "Yes."

Glenna Legarta, widow of another murdered journalist, said it was "infuriating," to hear testimony that Arroyo was aware of the threat of the Ampatuans.

"They knew about this. Even Gloria Arroyo knew ahead of time," she said.

Later at the trial, Justice Department coroner Ricardo Rodaje testified how Mangudadatu's wife, Jennalyn, bore 17 bullet wounds but was also hacked in her private parts before she died.

"These incisive wounds were inflicted while the victim was still alive," he said, describing his autopsy after the massacre.

He told the court that a double-edged blade had been used to inflict wounds on the "umbilical area, external genital area and right renal area."

Before the killings, the Ampatuans were close political allies of Arroyo, who armed and used them to help contain Muslim separatist rebels.

They were expelled from Arroyo's ruling party after the massacre and the government disarmed their private army.